The simplest system for transmitting a signal between communication devices is configured to transmit a signal from one communication device directly to another communication device. However, to guarantee the quality of wireless communications in this system, it is necessary, for example, to enhance transmission power or reduce a size of a wireless cell. In the first case, the power consumption increases. In the second case, since the number of base stations increases, the cost for configuring the system increases.
To solve the problem, a configuration of providing a relay station between communication devices is proposed and implemented. Described below is a system of providing a relay station RS between a base station BS and a mobile station MS, and performing a two-hop transmission between the base station BS and the mobile station MS.
FIG. 1A illustrates a classical relay system. In this relay system, the following four phases are necessary to bi-directionally transmit a signal between the base station BS and the mobile station MS.    (1) The base station BS transmits a signal to the relay station RS.    (2) The relay station RS transmits the signal to the mobile station MS.    (3) The mobile station MS transmits a signal to the relay station RS.    (4) The relay station RS transmits the signal to the base station BS.Therefore, the relay system requires four communication resources orthogonal to one another.
FIG. 1B illustrates an example of a conventional network coding system. In this relay system, the following three phases are necessary to bi-directionally transmit a signal between the base station BS and the mobile station MS.    (1) The base station BS transmits a signal S1 to the relay station RS through a link D1.    (2) The mobile station MS transmits a signal S2 to the relay station RS through a link D2.    (3) The relay station RS decodes the signals S1 and S2, performs exclusive-OR operation (XOR) for each bit of a set of the decoded data, and multicasts a result of the operation to the base station BS and the mobile station MS. The multicast packet is assigned a header, a CRC, and an FEC.
The system is sometimes called DF (Decode-and-Forward) relaying. The DF relaying requires three communication resources orthogonal to one another. As compared with the classical relay system illustrated in FIG. 1A, the throughput gain in two-hop communication may be achieved up to 33 percent at maximum in theory. When the hop count increases, the throughput gain of the DF relaying is achieved up to a double of the throughput of the classical relay system illustrated in FIG. 1A in theory.
In the conventional DF relaying, the relay station RS performs exclusive-OR operation for each bit. Then, the base station BS and the mobile station MS receive the signal “S1 XOR S2”, respectively. The base station BS can obtain the signal S2 by performing exclusive-OR operation on the received signal and the signal S1. That is, the base station BS can receive the signal S2 transmitted from the mobile station MS. Similarly, the mobile station MS can receive the signal S1 transmitted from the base station BS. Thus, a bi-directional communication is realized. The DF relaying is described in, for example, P. Larsson, N. Johansson, K. E. Sunell, “Coded bi-directional relaying”, the 5th Scandinavian WS on Wireless Ad-Hoc Networks (AdHoc' 05), Stochholm, Sweden, May 2005.
FIG. 1C illustrates another conventional network coding system. The relay system bi-directionally transmits a signal between the base station BS and the mobile station MS in the following two phases.    (1) A signal transmission from the base station BS to the relay station RS and a signal transmission from the mobile station MS to the relay station RS are simultaneously performed.    (2) The relay station RS amplifies interfered signals and multicasts the signals to the base station BS and the mobile station MS. At this time, the relay station RS does not decode the received signals.
The system is sometimes called AF (Amplified-and-Forward) relaying. The AF relaying requires only two communication resources orthogonal to each other. Therefore, the throughput gain of the AF relaying is double at maximum in theory compared with the classical relay system illustrated in FIG. 1A. The AF relaying is described in, for example, P. Popovsiki, and H. Yomo, “Wireless network coding by amplify-and-forward for bi-directional traffic flows”, IEEE Communications Letters, Vol. 11, No. 1, pp 16-18, January 2007.
However, the relay systems illustrated in FIGS. 1A through 1C have respective drawbacks as illustrated in FIG. 2. Since each phase is independently controlled in the classical relay system illustrated in FIG. 1A, the reliability of a link is high, and the flexibility (or freedom) of communications is also high. However, since the number of phases increases and the throughput is degraded in this relay system, the communication efficiency is reduced. The “flexibility of communications” refers to, although not univocally, the freedom in selecting a modulation scheme and a coding rate for a data transmission.
In the DF relaying illustrated in FIG. 1B, the reliability of a link is somewhat lower than in the system illustrated in FIG. 1A. The conventional DF relaying has a configuration in which XOR operation is performed on each bit of the data received from a pair of links, and the result of the operation is multicast. Thus, the same amount of data is to be transmitted through the pair of links. Therefore, when the communication quality of the pair of links is unbalanced, the data transmission of a link with higher communication quality is restricted by the data transmission of a link with lower communication quality.
The AF relaying illustrated in FIG. 1C has no communication flexibility. In addition, in an environment at a high noise level, the noise is also amplified in the relay station RS. Therefore, the communication performance is largely reduced. Although the problem may be reduced by implementing, for example, a DNF (Denoise-and-Forward) relay system, the configuration of the relay station is complicated and requires a higher cost.